1. Field of the Invention
The present invention relates to a method and apparatus for removing siloxanes which have a detrimental effect on the quality of products and are contained in silicon compound gases such as monosilane, disilane, halides thereof, silicon tetrafluoride, silicon tetrachloride and the like which are useful as semiconductor material gases; and also relates to an analyzing method and analyzing apparatus for analyzing semiconductor material gases such as monosilane, disilane, halides thereof, silicon tetrafluoride, silicon tetrachloride and the like for the concentrations of siloxanes mixed therein, which can be effectively used to evaluate the quality of the gases.
This application is based on Patent Applications Nos. Hei 9-36724 and Hei 9-46594 filed in Japan, the contents of which are incorporated herein by reference.
2. Background Art
Silicon compound gases such as monosilane, disilane, halides thereof, silicon tetrafluoride, silicon tetrachloride and the like have been widely used in the semiconductor industry as materials for epitaxial growth or growth of silicon oxide films and nitride films. However, the presence of impurities contained in the above-mentioned silicon compound gases can have a considerable influence on the quality of the produced semiconductors. Specifically, the presence of oxygen-containing impurities such as water (H.sub.2 O) or carbon dioxide (CO.sub.2) and the like can cause crystal defects which can lower the performance to the point of inadequacy. In particular, monosilane is known to have a tendency to react with water to produce detrimental siloxanes (H.sub.3 Si--O--H.sub.3 Si). For this reason, measures for preventing the intermixture of water have been taken.
Additionally, the siloxanes which have been produced and intermixed must be removed by purification in order to reduce the concentration to extremely low levels when using the above-mentioned silicon compound gases, and various methods have been proposed therefor. For example, an adsorption removal method using active alumina has been disclosed in Japanese Patent Application, Second Publication No. Sho 63-19443, an adsorption removal method using silica gel has been disclosed in Japanese Patent Application, Second Publication No. Hei 4-81523, and a removal method using getter metals has been disclosed in Japanese Patent Application, First Publication No. Hei 5-170405.
However, the above-mentioned conventional siloxane removal methods require the use of special agents such as getter metals, or the agents are difficult to activate favorably, so that they can contact monosilanes and cause the monosilanes to decompose and form hydrogen or disilanes. Additionally, since the adsorbents must be cooled to extremely low temperatures of below the ice point (0.degree. C.) in order to increase the removal effect, the costs can rise so as to inflate expenditures and the operations are difficult to handle. Furthermore, even with the proposed removal methods given above, the removal of siloxanes from silicon compound gases such as monosilane, disilane, halides thereof, silicon tetrafluoride, silicon tetrachloride and the like has not attained the levels (content: a few ppb or less) which are held to be desirable in the semiconductor industry.
Additionally, the silicon compound gases such as monosilane, disilane, halides thereof, silicon tetrafluoride, silicon tetrachloride and the like without the siloxanes which are not desirable for retaining the quality of products are put to effective use as material gases for epitaxial growth or growth of silicon oxide and nitride films in the production of semiconductors. However, even after removal of siloxanes, trace amounts of siloxanes which have a detrimental effect of the quality of semiconductors remain mixed into the silicon compound gases. Therefore, due to increases in the scale of integration of semiconductors, the elimination of this siloxane content and the development of silicon compound gases of higher purity has been desired. In correlation therewith, the development of analysis techniques having lower minimum limits of sensitivity (a few ppb or less) for detecting siloxanes in silicon compound gases has become an essential prerequisite. That is, the development of high-quality material gases for the production of semiconductors containing extremely minute amounts of siloxanes is possible only after the establishment of an analysis method having a lower minimum limit for detection of siloxanes of only a few ppb or less as a method for analyzing gases to evaluate their quality.
Conventionally, silicon compound gases have been analyzed for siloxane content by using thermal-conductivity-detecting gas chromatography (hereinafter referred to as "TCD-GC") as a detection means. With this method, samples are analyzed by separating the components using as a separation column a stainless steel tube which is filled with Porapack P or Porapack Q (both products of Waters Corp.) composed of porous macromolecular agents such as divinylbenzene (DVB). According to this method, the detection sensitivity for siloxanes is merely 0.5-1 ppm (500-1000 ppb), which leaves impossible the detection of a few ppb which is required as described above.
Since the conventional TCD-GC analysis methods require the separation column to be heated to a temperature of 70-80.degree. C. in order to conduct an analysis for siloxanes, the present inventors pinpointed the heating temperature as the likely cause of the minimum detection limits, and therefore measured the changes in the siloxane content by guiding gas discharged from the separation column to a mass spectrometer while changing the column temperature. The results are shown in FIG. 13. The separator which was used was Porapack P, the gas which was used was monosilane, of which 5 cc were collected for analysis at each temperature.
As is clear from FIG. 13, the amount of siloxanes from the column changes according to the temperature of the separation column, even if the same gas is analyzed. That is, when the temperature is high, the amount increases, so that the content of siloxanes was confirmed to be approximately 700-1200 ppb when the separation column had a temperature of 70-80.degree. C. at which the gases are analyzed in TCD-GC, and was confirmed to achieve approximately 7000 ppb at a temperature of 100.degree. C. On the other hand, at the lower temperatures, the content was lower, e.g. the content at 60.degree. C. was approximately 300 ppb, and the content at 40.degree. C. was approximately 60 ppb. This phenomenon is believed to be caused by reactions between trace amounts of residual water adhered to the macromolecular separator in the separation column and the silicon compounds which are introduced, which results in the production of siloxanes, of which the rate of production increases as the temperature rises.
Additionally, when analyzing silicon compound gases for siloxanes according to a conventional TCD-GC technique using heated separation columns, the amount of siloxanes extracted from the column changes depending on the temperature of the separation column, so that it is difficult to determine the true value. Furthermore, when the separation column used in this analysis method has a heating temperature of 70-80.degree. C., the content of siloxanes is 700 ppb or more, which makes it impossible to detect the siloxane content in silicon compound gases in minute amounts of a few ppb, which is expected to become an essential requisite in this field.